1. Field of the Invention
The present invention relates to a frequency channel selection method for a radio communication system and, more particularly, to a frequency channel selection method for a radio communication system in which a plurality of radio stations communicate with each other.
2. Description of the Prior Art
In a conventional radio communication system such as a car or cellular telephone system, the service area is covered by a plurality of basic stations, and the same frequency channel (to be simply referred to as a channel hereinafter) is repeatedly used between basic stations which cause no radio interference, thereby realizing the effective use of frequencies. Such a system is called a cellular system. In the cellular system, when a given basic station and a given movable station start to communicate with each other, the basic station measures the carrier to co-channel interference ratio (to be referred to as a C/I hereinafter) of each of unused channels to select a channel that has a lower interference wave power and a C/I higher than a predetermined C/I. In general, a desired carrier power (C) is obtained by measuring the reception power of a control channel ensured independently of a communication channel, and an interference wave power (I) is obtained by measuring the reception power of the communication channel to be selected, as in the channel assigning method disclosed in Japanese Unexamined Patent Publication No. 3-174827.
In a radio packet communication system like the one disclosed in Draft Standard IEEE802. 11, P802. 11D2.0, Wireless LAN Medium Access Control and Physical Layer Specifications, one radio link is shared by one basic station and a plurality of movable stations which are connected to a wire network. Channels are permanently assigned to the respective basic stations on the basis of propagation environments and predicted communication traffics. The user of a given movable station manually sets a channel to be used on the radio link to which the given station belongs. Alternatively, a channel is set on the basis of a control signal which is constantly or periodically transmitted from a basic station. Since the output of each basic station is fixed, the coverage of the service area is also fixed.
As described above, in the conventional radio communication system, to select a channel, a control channel other than a communication channel is required, or a control signal must be periodically transmitted. For this reason, frequencies cannot be effectively used.
In addition, as described above, in the conventional radio packet communication system, channels are permanently assigned to the respective basic stations. For this reason, when a plurality of basic stations are to be installed, channel assignment design must be performed in advance to assign specific channels to specific basic stations. Such assignment of channels cannot be automatically performed. Furthermore, since the output of each basic station cannot be automatically changed depending on communication states, communication is sometimes performed on a transmission power that is higher than required, resulting in an increase in interference wave power in other radio stations.